Free-piston electric generators are already known which, according to the Diesel principle, supply medium-power turbo-alternators with hot gases at about 450.degree. C. and an average pressure of about 3.times.10.sup.5 pascals. Such generators have an overall efficiency of 35-36%, which is comparable to a conventional Diesel alternator system.
More recently there have been disclosed horizontal free-piston engines having one or two pistons in opposed movement, carrying annular magnetic members which reciprocate between induction coils provided in the cylinder associated therewith, thereby generating electric current in the coils. In such engines the compression of the Diesel fuel mixture at a suitable pressure is ensured by the return of the piston or pistons which is produced by the energy stored in springs or hydraulic accumulators of appropriate design as to their function and to the maintenance of the energy storage capacity over a period of time.
U.S. Pat. No. 3,105,153 and French printed patent application No. 2,204,068 both relate to free-piston electric current generators. In the first the free piston is suspended by oscillating cross member and in the second the free piston may be horizontal. Such apparatus have closed cylinders. Moreover, they require either an explosion in a second combustion chamber (U.S. Pat. No. 3,105,153) or metal springs (French printed patent application No. 2,204,068) to ensure the return of the piston or pistons and the compression of the gaseous mixture necessary for the following explosion.